This invention relates to a method of coating a metal object with a fluorinated polymer, and more specifically, it relates to thermally bonding a toner transfer-resistant fluorinated polymer layer onto metal fuser rolls for electrophotographic copy machines.
In the process of electrophotographic copying a light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic particles, commonly referred to as toner. The visual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support, such as a sheet of plain paper, with subsequent affixing of the image thereto. Toners are well known in the art and may be of various types.
In order to affix or fuse electroscopic toner material onto a support surface permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent into the fibers or pores of support sheets or members or otherwise upon the surface thereof. Thereafter, as the toner material cools, it solidifies and becomes bonded firmly to the support in the well known manner.
Several approaches to thermal fusing of electroscopic toner images onto a support have been described in the prior art and include providing the concomitant application of heat and pressure, as by a roll pair maintained in pressure contact, a flat or curved plate member in the pressure contact with a roll, a belt member in pressure contact with a roll, and the like. Heat may be applied by heating one or both of the rolls, plate members or belt members. The fusing of the toner takes place when the proper combination of heat, pressure and contact time are provided, the balancing of these parameters being well known in the art and varying according to various factors which must be independently determined for each particular situation.
During operation of a typical system for thermal fusing of electroscopic toner images onto a support in which at least one fuser member, such as a fuser roll, plate, or belt, is heated, the support member to which the toner images are electrostatically adhered is moved through a xe2x80x9cnipxe2x80x9d formed between the members with the toner image pressure contacting the fuser member. Thus the toner images are heated within the nip. By controlling the heat transfer from the fuser member surface to the toner such that the surface temperature does not rise above the xe2x80x9chot offsetxe2x80x9d temperature at which the toner particles in the image areas liquefy, virtually no offset of the toner particles from the support (e.g., a copy sheet) to the fuser member is experienced under normal conditions. If the toner melts due to exceeding the hot offset temperature to an extent that the cohesive forces holding the viscous toner mass together are less than the adhesive forces tending to adhere the mass to a contacting surface such as a fuser roll, fuser belt, or fuser plate, the toner is prone to split and offset to such surface.
Occasionally, however, toner particles will be xe2x80x9ccold offsetxe2x80x9d to the fuser roll, that is, when the surface temperature is below the xe2x80x9chot offsetxe2x80x9d temperature. Some reasons for cold offsetting are imperfection in the properties of the surface of the roll, insufficient adherence of the toner particles to the copy sheet, insufficient strength of the electrostatic forces which normally hold toner particles to the copy sheet, and, in certain cases, the reactivity of the toner material itself. In such a case, toner particles may be transferred to the surface of the fuser member with subsequent transfer to the backup member which provides pressure contact during the period of time when no copy paper is in the nip.
It is known in the art that undesirable offsetting of the toner from the support can be effectively prevented during toner fusing operations by interposing a barrier layer of a heat and toner adhesion resistant material between the fuser member and the support. In many of these pressure contact, heat fixing systems, the fuser member is a metal fuser roll and the barrier layer is applied as a covering on the outer surface of this fuser roll. As an example of the heat resistant, release material for the fuser roll, there are well known materials such as polytetrafluoroethylene, silicone rubber, fluorocarbon elastomers and the like.
Perfluorinated polymer compositions are preferred for the heat resistant fuser roll covering largely because they exhibit superior low adhesion to toner. Unfortunately, this xe2x80x9cnon-stickxe2x80x9d property also makes such polymers difficult to durably adhere to the metal which usually forms the core of the covered fuser roll.
The conventional method of putting a coating of the preferred, toner adhesion resistant polymers onto a metal fuser roll core involves providing the polymer in fine particle size. The polymer is either dispersed or dissolved in an appropriate medium to form a fluid that usually includes additional adhesive components. The fluid is sprayed onto the core and the adhesive components are cured to form a bond between the metal and the polymer. To apply a coating of a highly fluorinated polymer, which is a much desired type, it had been found that adhesion to a metal substrate is promoted by first etching the polymer and then utilizing adhesives, among other steps. Etching compositions for fluorocarbon polymers are described in such patents as U.S. Pat. Nos. 2,789,063 and 2,809,130.
Fuser rolls having a fluorinated polymer cover affixed by etching and adhesives and the process for making such rolls suffer from several drawbacks. One limitation is that the bonds produced by the adhesive after etching often cannot withstand the high temperatures of electrophotographic processing. Consequently, the heat resistant fluorocarbon barrier layer soon peels off or becomes so loosened from the surface of the metal core as to be rendered prematurely useless. Other problems arise from handling the fluorocarbon polymer as a powder dispersed in a fluid medium. Such problems often relate to the need to properly proportion the polymer and other ingredients, the need to protect against potential hazards and waste caused by spills of raw materials during processing, and the cost of providing, operating and maintaining equipment used to process and apply the coating fluid. It is especially problematic that the texture of the surface of a fuser roll with spray applied and adhesively bonded fluorinated polymer coating is too rough as directly produced for electrophotographic copying and like applications. To achieve the desired degree of smoothness, such a fuser roll normally is burnished by application of sufficient heat to soften the polymer for a duration effective to cause the microscale peaks and valleys to level.
U.S. Pat. No. 4,258,089 provided a method of applying fluorinated polymer to a metal fuser roll surface that sought to overcome these drawbacks by eliminating the use of etchant and additional adhesive material altogether. Generally, this method called for first cleaning the surface of the substrate and next heating the substrate to a temperature high enough that the polymer to be coated thereon softens, melts or gels. The polymer is contacted with the substrate, and then pressure contact between the polymer and substrate can be applied with force effective to form a bond. The polymer could be to a roller in tube form. In such a case, the polymer tube could be heated prior to applying the pressure contact.
The method of the ""089 patent goes a long way toward improving the art of coating a metal fuser roll with a fluorocarbon polymer. However, the apparatus disclosed to coat fuser rolls is difficult to implement with high productivity. The ""089 patent suggests heating the roll to be coated by a heating element inserted inside the roll and applying the bonding pressure with a second roll rotating in contact with the coated roll. The second roll is continuously cooled so that the coating does not transfer onto it from the fuser roll intended to be coated.
It is desirable to have a method of coating metal core fuser rolls with toner adhesion-resistant fluorinated polymer that overcomes the disadvantages mentioned above. That is, it would be welcome to have a coating method that provides a permanent bond between the polymer and the metal core without utilizing etchants or additional adhesives. Further, it is wanted to affix a fluorinated polymer coating without spraying a powder dispersed in a fluid medium so as to provide ease and simplicity of processing and production of suitably smooth surface coatings without having to resort to post process burnishing. It is also desired to have a method that avoids application of external force to press the polymer against the core while the coating is affixed thereto.
Accordingly, the present invention now provides a method of making a fuser roll comprising the steps of
(a) providing a cylindrical core having a core outer diameter,
(b) providing a slightly expandable, cylindrical fluorinated polymer sleeve having a sleeve inner diameter smaller than the core outer diameter, the fluorinated polymer having a thermoplastic softening temperature,
(c) inserting the core into the fluorinated polymer sleeve to form an unbonded assembly,
(d) while maintaining temperature below the thermoplastic softening temperature, subjecting the unbonded assembly to a subatmospheric pressure for a duration effective to remove substantially all gaseous components from between the core and sleeve,
(e) while maintaining the unbonded assembly at the subatmospheric pressure, heating the unbonded assembly to an elevated temperature above the thermoplastic softening temperature for a duration effective to fuse the fluorinated polymer to the core, and
(f) cooling the assembly to a temperature below the thermoplastic softening temperature, thereby affixing the fluorinated polymer to the core with a bond.